Apparatus for extruding synthetic plastic resins at low temperatures

ABSTRACT

A method and apparatus for manufacturing synthetic resin extrusion products by controlling the temperature of the resin to within + OR - 10*F. of a desired point as it is formed into a homogeneous mass and extruded. This control is accomplished by providing one temperature sensing thermocouple in the barrel of the extruder, another temperature sensing thermocouple in the heating/cooling jacket mounted on the outside of the barrel, and by providing electrical control circuitry which averages the temperature readings of the two thermocouples and controls the supply of energy to the heating/cooling jacket in accordance with the difference between that average and the desired temperature to which the controller is set. Included as well is the novel synthetic resin extrusion article obtained by extruding the resin upwardly into a tubular parison using this low temperature process and stretching the parison axially before blow molding it.

United States Patent 1191 Waterloo 1451 Aug. 7, 1973 [75] Inventor:William C. Waterloo,York, Pa.

[74] Asignee: Graham Engineering Corporation,

York, Pa.

[22] Filed: Sept. 21, 1970 [21] Appl. No.: 73,870

1521 us. (:1 259/191, 165/30, 259/9, 259/010. 18, 425/144 511 1111. c1.82% 1/06, 82% 3/00, HOSb 1/02 [58] Field of Search 259/185, 191, 192,259/193, 9, 10, DIG. 18; 425/144; 236/15 B, 78 B; 165/30 [56] 1References Cited UNITED STATES PATENTS 3,317,953 5 1967 Stroup et al.259 191 x' 3,143,167 11 1964 1 Vielh 236/78 B x 3,625,421 12/1971(itilllSOIl 236/78 B x 3,315,592 4 1967 Haakc 236/788 2,508,988 5 1950Bradley 425 144 x 3,447,790 6 1969 ROSSelZll... 263/28 3,431,599 3 1969Fogelberg 1. 425/144 x 3,129,459 4/1964 Kullgren et al. 425 144 xPrimary Examiner-John Petrakes Assistant Examiner-Alan l. CantorAttorney-C. Hercus Just [57] ABSTRACT A method and apparatus formanufacturing synthetic resin extrusion products by controlling thetemperature of the resin to within ilOF. of a desired point as it isformed into a homogeneous mass and extruded. This control isaccomplished by providing one temperature sensing thermocouple in thebarrel of the extruder, another temperature sensing thermocouple in theheating/cooling jacket mounted on the outside of the barrel, and byproviding electrical control circuitry which averages the temperaturereadings of the two thermocouples and controls the supply of energy tothe heating/cooling jacket in accordance with the difference betweenthat average and the desired temperature to which the controller is set.lncludedl as well is the novel synthetic resin extrusion articleobtained by extruding the resin upwardly into a tubular parison usingthis low temperature process and stretching, the parison axially beforeblow molding it.

7 Claims, 6 Drawing Figures PAIENTEDM 7 ms snmmu INVENTOR.

W l-UAM C. Ware 240 A 7'TOEMEY PATENIEW SHEET & 0F 4 APPARATUS FOREXTRUDING SYNTHETIC PLASTIC RESINS AT LOW TEMPERATURES BACKGROUND OF THEINVENTION This invention relates to the extrusion of synthetic resinsand more particularly to a novel method and apparatus for extruding suchresins under closely controlled temperature conditions. It also concernsa novel extrusion product made by this controlled temperature process. A

Heretofore, extrusion has been widely used in the plastics industry forcontinuously melting, blending, forming and solidifying plastics into adesired shape. Extruders are often used, for example, in blow moldingoperations to form tubular parisons which are then blown in a mold toform the desired shape. A typical extruder includes a rotating screwenclosed in a cylindrical barrel and a heating/cooling system (i.e.,heat energy transfer system) in several zones surrounding the barrel.When properly combined these elements produce a fully plasticatedthermally homogeneous melt'which is forced i.e., extruded through a die.

It is known to provide the barrel with heating/cooling systems of castaluminum which contain electrical resistance elements to supply heat andwhich also contain embedded tubes to carry cooling water to remove theexcess heat from the barrel.

The heating is required to bring the resin up to its meltingtemperature; the cooling is needed to carry away excess heat whicharises from any source and particularly that which comes from themechanical working of the resin by the screw.

All temperature control methods and systems have to cope with the factthat in any plastics extrusion apparatus the barrel through which theresin passes and the heating/cooling devices which are affixed to it actas heat sinks and absorb or give off a certain amount of heat beforerising or falling in temperature. Thus there is a delay between the timeheating or cooling is begun and the time the desired temperature reachesthe resin inside of the barrel.

In prior systems this lag in bringing the barrel to the desiredtemperature has resulted in barrel temperatures which swing overrelatively large ranges, for example, from about 2550F. about ,a desiredpoint. This has meant that to ensure that some of the resin doesnt getbelow its melting point the swings of the barrel temperature have had tobe kept further above the melting point of the resin than is desirable.Keeping the resin that hot, however, increases the risk of burning itparticularly when the extruder is shut down overnight and among otherthings makes the manufacturing of products from the melt more difficultbecause they have to be cooled down further to become solidified.

It is therefore one object of the invention to provide an improvedmethod and apparatus for controlling the temperature of the resin towithin a narrow range about a set point as it is being plasticated andextruded.

Another object is to provide an improved synthetic extrusion product bythis novel method.

A further object is to provide a method and apparatus for producing blowmolded products more efficiently in less time and using less resin.

Still another object is to provide a novel method and apparatus whichwill control the temperature of the resin to within a range of about ilOF. with respect to a desired set point while it is being plasticatedand extruded.

Further, other and additional objects of the invention will becomeapparent from the Summary, Detailed Description, Claims and drawingswhich follow hereinafter.

BRIEF SUMMARY OF THE INVENTION In one aspect the invention comprises amethod of controlling the temperature of synthetic plastic resins abouta set point as they are worked into a thermally homogeneous melt andthen extruded, comprising the steps of sensing the temperature withinthe extruder at two different radial distances from the axis of theresin passage through the extruder and controlling the ex change of heatwith the extruder barrel in accordance with both temperatures. Morespecifically the method includes sensing the temperature of the extruderbarrel in a well on its periphery at one radial distance from a point onthe axis of the resin passage through the bar rel, sensing thetemperature in a heat exchange device mounted on the barrel at a greaterradial distance from that point and controlling the amount of heatexchanged between the barrel and the device according to the differencebetween the average of the two temperatures being sensed and a desiredtemperature (i.e., set point). This technique has several advantages,one of which is that it enables the heating and cooling of the extruderbarrel to begin before the thermocouple in the barrel indicates a changeis needed. The change is called for just early enough in the cycle tobring the heat exchange unit and the barrel-to the required temperaturelevel about the time that heat needs to be exchanged. This enables theresin to be kept within about 2-5F. of a desired set point and thuscloser to the melting point of the resin than heretofore possible.

In another aspect the invention includes apparatus for controlling thetemperature of synthetic plastic resins about a set point as they areworked into a thermally homogeneous melt in an extruder comprising atemperature sensing device mounted on the barrel of the extruder, asecond temperature sensing device mounted on a heat exchange unit ingood thermal contact with the exterior of the barrel and means includingcontrol circuitry for adding or subtracting heat to and from the unitaccording to the signals received from these devices. The temperaturesensing devices are preferably thermocouples, mounted in wells in thebarrel and the heat exchange units.

In a further aspect the invention includes the synthetic resin articlewhich is produced using the method of this invention. More specificallyit comprises the blow molded container orother product which is producedfrom a blown-in-the-mold parison tube extruded in accordance with theinventive method. Such containers are greatly improved over blow moldedproducts made from extruded parisons whose temperature is not kept closeto the fusing temperature of the material. For example, blow molded highdensity polyethylene containers made from the low temperature parisonextruded in accordance with the method of this invention have a greaterimpact strength than products from a similar parison extruded at highertemperatures. In addition, when the resin used is clear, extruding theparison according to this method improves the clarity of the blow moldedproduct. Also, it enables the products to be made faster because lesscooling is required to solidify the melt.

Further, other and additional aspects and advantages of the inventionwill become apparent from the description and claims which followhereinafter.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical elevation view of apreferred embodiment of an extruder apparatus according to the inventionpartly broken away.

FIG. 2 is a larger scale vertical elevation view of one section of aheat exchange unit for the barrel of the extruder of FIG. 1 showing theheating and cooling features of the unit.

FIG. 3 is a left profile view of the temperaturesupplying unit shown inFIG. 2.

FIG. 4 is a large scale cross sectional view of the barrel section ofthe preferred embodiment taken on line l-l of FIG. 1, but illustrated ona larger scale than employed in that figure, illustratingdiagrammatically a preferred temperature control means for the heatexchange unit of FIGS. 2 and 3.

FIG. 5 is a vertical sectional view of a parison extruder head and ablow-molding mold clamped about the extruded parison in accordance withthe invention.

FIG. 6 is a small scale diagrammatic side elevation view of the parisonextruder head and Ferris wheel on which the molds are mountedillustrating the relative positions of these elements in accordance withthe invention.

DETAILED DESCRIPTION Referring now to FIG. 1, a preferred embodiment ofthe apparatus according to the invention includes an extruder 10 and asupply hopper 12 positioned at one end adjacent the inlet 14 by whichgranular or powdered thermoplastic synthetic resin is delivered to theentrance of the horizontal barrel 16 of the extruder 10. A screw 18 forplasticating the resin and advancing it through the barrel is mountedwithin the bore of the barrel 16.

The end of the screw 18 adjacent the inlet 14 of the system is connectedto suitable drive means, not shown, for rotating the screw 18 at adesired rate and in a di rection to cause the spiral flights 20 on thescrew to work and plasticate the resin and propel it toward thedischarge end 22 of the barrel 16. The barrel 16 is suitably supportedin a stationary manner within a housing 24 of suitable shape andconstruction. The discharge end of the barrel delivers fused or meltedresin to an extruder head 25.

To control the temperature of the resin in the barrel, a plurality ofheat exchange units 26 are mounted one after the other next to eachother around and along the length of the barrel 16. Each of these unitsis constructed to furnish heat or cooling media to the barrel for thepurpose of exchanging heat with the barrel in a specified area along itslength to insure control of the temperature of the resin in that areawithin a narrow range about a desired set point. The heat exchanged withthe barrel is very carefully and precisely controlled by means describedhereinafter so that, under preferred conditions of operation, the resindoes not vary more than 5F. or at the most 10F. above or below a desiredset point in each controlled zone.

The temperature-supplying units 26, comprise semicylindrical castings 28(see FIG. 3) which are made of high thermal conductivity metal, such asfor example, certain aluminum alloys, which are highly suitablematerials for forming castings 28. Each casting also preferably isprovided with heating means as well as cooling means. The heating means(see FIGS. 2 and 3) comprise a sinuous coil 30 of electrical resistancemeans of suitable capacity. One type of electrical resistance meansavailable for such purposes is sold under the tradename Calrod. The coilis provided at opposite ends with terminals 32 by which it iselectrically connected to a controlled source of electricity. Details ofthe control for this source means 50 are described hereinafter.

The castings 28 also are each provided with a sinuous arrangement of aunitary conduit 34 which has an inlet end 36 and an outlet end 38. Ineach casting the end 36 and end 38 respectively are connected to supplyconduits, not shown, carrying liquid coolant. Conduit 34 lies adjacentthe radially inner surface of casting 28 and in this way keeps thecooling medium closer to the barrel than the electrical coil 30 which isnear the radially outer surface of the casting. Preferably, distilledwater from a closed system is used as the liquid coolant in order tomaintain minimum corrosion or deposit upon the inner walls of thesinuous conduit 34. The use of water as a coolant also has the advantagethat if enough heat is added it will cool by boiling without rising intemperature thus utilizing the heat of vaporization of the coolant tocool effect by absorbing heat from the casting 28. In such a closedsystem, details of which are not illustrated, a suitable condenser orheat exchanger, not shown, may be employed to condense any of the fluidwhich has been vaporized and also reduce the temperature of thecondensed fluid for reuse as a coolant.

The two semi-cylindrical castings of each of the units 26 are mounted onthe barrel by conventional means. The inner surface 40 of each of thecastings is shaped to closely complement the exterior surface of thebarrel 16 and is in good thermal contact with this surface for efficientpassage of heat by conduction between the heat exchange units 26 and thebarrel 16.

Each of these units 26 has a temperature control system associated withit which controls the addition and subtraction of heat to and from thebarrel and maintains the temperature of the resin with a very narrowrange. For example, using high density polyethylene, the meltingtemperature of which is between 330420F., the apparatus of the inventionis adapted to bring the resin to a temperature of 380F. and to keep itvery close to that temperature during plastication and extrusion. Otherresins may be worked and extruded at different temperatures.

Referring now to FIG. 4, between the ends of each of the portions of thebarrel l6 enclosed by units 26, the barrel is provided with a shortradially inwardly extending well 42 of limited diameter. The depth ofthe well is preferably about half the thickness of the wall of thebarrel 16. One of the two castings 28 in each heat exchange unit 26 alsois similarly provided with a radially inwardly extending. well 44 alsoof similar diameter and depth, the depth preferably being approximatelyhalf the thickness of the casting 28.

In these radial wells 42 and 44 are mounted temperature-sensing elements46 and 48. The preferred type of such element is a thermocouple ofappropriate rating and capacity. The electrical leads from eachthermocouple are connected to the controller 50 with the leads from thepositive side of the thermocouples 46 and 48 connected to a positiveterminal 52 on controller 50 and the leads from the negative side to anegative terminal 54. The controller is a commercially available deviceand is provided with a null balancing circuit, not shown, whichautomatically averages the temperatures indicated individually by thetemperature-sensing elements 46 and 48. The device also compares theresulting average with a predetermined temperature or set point which isset into the controller 50 by adjusting a variable potentiometer in thenull balance circuit. It generates a signal of one sign to add heat tothe unit 26 if the circuit is off balance in one direction and a signalof opposite sign to add cooling if it is off balance in the oppositedirection. When the circuit is in balance it puts out no signal andneither heat nor cooling is added to the unit.

Each heat exchange unit 26 has its own controller 50 and adds orsubtracts heat to or from the barrel automatically in response to asignal from its controller 50. At any given time depending on acomparison of the average temperature reading with its set point some ofthe units 26 may be adding heat, some may be doing nothing, and some maybe subtracting heat. Each unit is controlled completely independently ofthe others. When a decrease in temperature is required at one locationthe controller 50 associated with the heat exchange unit in that areaoperates a solenoid valve 76 to begin the flow of cooling water throughthe unit 26.

Cooling is stopped when the controller operates the valve 76 in theother direction to shut the water fiow off. When heat is requiredelectrical energy is fed to the heaters 30 and is shut off again when abalance is regained. One advantage of the method and apparatus accordingto the invention is that all of the heat exchange units including theone in the extruder head are temperature controlled so closely that thetemperature never varies more than i 10F. around its set point. In fact,the apparatus according to the invention is adapted to keep the averagetemperature indications of the two thermocouples associated with eachcontroller to within about i 5F. of the temperature point to whichthatcontroller is set. Because of this accurate control of the resintemperature the resin is worked out and extruded at a lower temperaturewith the resulting benefit that the quality of thefinal product isimproved and the time needed to cool it is lessened. This reduction inrequired cooling time is important because it can and often doescomprise a majority of the total time required to make, for example, ablow molded product. In addition. by controlling the temperature soclosely the chances of burning" the resin and otherwise preventingimpairment of the visual and/or physical properties of products moldedfrom the fused resin are improved. This advantage is particularlysignificant in shutting down the extruder overnight because thetemperature control system of the invention often permits the extruderto be shut down with the resin in it without burning the resin.

The controller 50 is electrically connected via its ter' minals 56 and58 to a conventional power pack 68 which uses silicon controlledrectifiers (SCRs) to control the amount of power fed to the load inaccordance with electrical signals from the controller. The size andsign of the signal from the controller determines how much power thepack 68 delivers to the coils 30 which are electrically connected to thepower pack. Terminals and 72 of controller 50 serve to trigger an alarmcircuit including an alarm 74, if the temperatures sensed get too faraway from the desired set point. The controller itself gets power fromconductors 60, 62 connected to a conventional I 10 volt AC sourcethrough terminals 64, 66.

In the preferred apparatus according to the invention there are six heatexchange devices, five of which are clamped around the barrel 16 inclosely spaced relationship along its length. Of these five the oneclosest to the inlet 14 surrounds the feed zone and normally adds heatduring the operation of the extruder in order to bring the resin up tothe desired temperature. The next two toward the discharge end of thebarrel 22 surround the compression zone of the barrel and during normaloperation of the extruder neither add nor subtract much heat. Thus theyoperate in a relatively steady state condition. The final two of thesefive heat exchange units lie in the metering zone closest to thedischarge end of the barrel and normally remove the excess heatgenerated by mechanical working of the resin by the screw 18. Thetemperatures to which the controllers 50 are setfor each of the heatexchange units in these three zones, are preferably different with thehighest temperature set point being at the discharge end 22 of thebarrel and the lowest at the inlet end 14.

In addition, there is a heater (not shown) and a controller 50associated with the extruder head 25. This heateris an electricalresistance device alsocontrolled by a null balance circuit as are theothers. Since the extruder head 25 contains no cooling coils, however,the circuit only feeds power to the heater when off balance in onedirection. Nothing happens when it is off balance in the other. Normallysince the' resin is not worked to any significant degree in the extruderhead cooling is not necessary. This sixth and last controller 50normally has no averaging function since there is only one thermocouplein the extruder head.

In blow molding tubular parisons extruded in accordance with the methodof the invention the temperature of the resin is extruded atatemperature sufficiently above its fusing point that when separatedinto streams to flow around the mandrel it is warm enough to becomefused again without a weld seam within the extruder head 25, exemplarydetails of which are shown in FIG. 5. The melted resin enters head 25through an inlet 78 and passes around the mandrel 80 which encloses ahollow die pin stem 82. The position of the mandrel 80 is adjustablealong its lengthwise axis relative to the walls of the cavity 84.. Itsposition in the cavity controls the volume of resin delivered to thedischarge end 86 of the head 25 in a given length of time. The outer endof the stem 82 comprises a hollow die pin 88 which is movablelongitudinally relative to the discharge orifice 86 of the head 25.Movement of the die pin 88 is preferably controlled by a programmingcontrol unit 90, diagrammatically shown in FIG. 6, to vary the crosssectional thickness of the parison 92 in accordance with theconfiguration of the (see FIG. 5) product to be blown.

When blow molding a product using the method according to the inventionthe parison 92 isextruded upwardly from the head 25 and introduced intoa cavity 94 defined by two transversely movable mating mold halfsections 96. These mold sections are mounted on a rotatable Ferris wheel98, which rotates clockwise as shown by the directional arrow in FIG. 6and includes means whereby the mold halves are clamped around theparison tightly pinching it at the top and bottom of both halves. As thewheel rotates the clamped mold halves move upwardly and away from theextruder head 25 and pull the parison upwardly. By setting theperipheral speed of the wheel and thus that of the mold sections at alevel which is slightly higher than the speed with which the parison iscontinuously extruded. The parison is stretched lengthwise by one moldbefore the succeeding mold is clamped around it. This stretching orientsthe molecules of the resin axially with respect to the parison and tothe mold which clamps about it and improves the characteristics of theblown product as described more fully hereinafter.

When the mold sections 96 are clamped around the parison 92 it closesoff the ends of the parison within the mold. Then a blowing needle 100actuated by a conventional mechanism, not shown, is driven into a wallportion at the upper end of the parison (see FIG. and air is blownthrough the needle into the parison to inflate it until its walls comein firm contact with the walls of the mold sections 96 defining the moldcavity 94. Cooling fluid passing through these mold sections solidifiesthe product thus formed and when sufficient cooling has beenaccomplished the mold halves are separated again and the blown productremoved.

In this process the variations in the wall thickness of the parison arecontrolled in a timed reiation to the presentation of the mold sectionsto the parison so that when the parison is clamped by the mold sectionsthe portion of the parison which may form, for example, a neck of theblown product, has one wall thickness and the body of the product, whichoften is blown up to a much greater degree than the neck, anotherthickness.

The blowing air which is introduced into the hollow molded object 102will also facilitate the cooling of the molded product and if desired,may be refrigerated. As a result of the blowing of the parison to expandit circumferentially, the resin of the resulting product is given amolecular orientation in the transverse direction. This is, of course,combined with the molecular orientation of the resin in the lengthwisedirection accomplished by stretching the parison, as described above,with the result that the resin in the product is biaxially oriented.This greatly increases the tensile and compressive strength of theproduct and thereby results in permitting the production of hollowproducts having thinner wall sections without a loss in strength. Inthis way the process is more economical since less resin is required foreach product.

From the foregoing, it will be seen that the present invention producesa superior product which not only has a greater impact strength due tobeing extruded at a lower temperature than has heretofore been possible,but when blow molded also has a greater tensile and compressive strengthfor a given wall thickness than products made on conventional blowmolding equipment, due among other reasons to the bi-axial orientationof the molecules of the resin.

To prevent collapsing and possible cohesion of the parison walls whenthe mold sections close upon it and pull it away from the parison head,air, known as parison air, is introduced through the hollow core of thedie pin 88 under sufficient pressure to keep the walls apart. Ifdesired, temperature of the air introduced into the hollow parison maybe kept low to aid in cooling the parison.

While the invention has been described and illustrated in its severalpreferred embodiments, it should be understood that the invention is notto be limited to the precise details herein illustrated and describedsince the same may be carried out in other ways falling within the scopeof the invention as illustrated and described.

What is claimed is:

1. Apparatus for extruding synthetic resin comprising in combination, abarrel having resin inlet and discharge means respectively adjacentopposite ends thereof, resin propelling screw means within said barrelfor mechanically working and plasticating said resin, heat exchangeunits mounted on the outside of the barrel and connectable to sources ofheating and cooling media to control the temperature of resin thereinand operable selectively to heat and cool the same, means comprising asignal generating temperature-sensing device extending into the barrel apredetermined radial distance from its axis to sense the temperature ofthe resin within the barrel, a second signal generatingtemperature-sensing device mounted upon the said heat exchange units andoperable to sense the temperature thereof, and means including controlcircuitry connected to both said temperature-sensing devices andoperable to average the indications thereof relative to a desired setpoint of temperature and control the delivery of heat or cooling to saidheat exchange units for transfer of the effect thereof to said barrel asrequired to produce a desired temperature in the resin discharged fromsaid barrel in accordance with the signals generated by said devices.

2. Apparatus according to claim 1 wherein the heat exchange deviceincludes both heating means and cooling means and the means includingcontrol circuitry includes means for energizing the heating meanswhenever the indicated temperature average is below the set point by apredetermined amount and for energizing the cooling means whenever thisaverage is above the set point by a predetermined amount.

3. Apparatus according to claim 2 wherein there is a well in the radialperiphery of the barrel and the first temperature sensing devicecomprises a thermocouple mounted in the well, the heat exchange deviceis mounted on the barrel, there is a well in the periphery of the heatexchange device and the second temperature sensing device comprises athermocouple mounted therein and said control circuitry includes a nullbalance circuit.

4. Apparatus according to claim 3 wherein each of the thermocouples lieat different radial distances in a single plane through andperpendicular to the barrel.

5. Apparatus according to claim 4 wherein the control circuitry isvoltage regulating circuitry whereby the amount of energy fed to theheating means increases as the difference between the average indicatedtemperature and the set point becomes greater.

6. Apparatus according to claim 1 wherein the heat exchange unitsinclude heating means and the means including control circuitry includesmeans for energizing the heating means whenever said average is lowerthan a predetermined level.

7. Apparatus according to claim 1 wherein the heat exchange unitsinclude cooling means and the means including control circuitry includesmeans for energizing the cooling means whenever said average is higherthan a predetermined level.

2. Apparatus according to claim 1 wherein the heat exchange deviceincludes both heating means and cooling means and the means includingcontrol circuitry includes means for energizing the heating meanswhenever the indicated temperature average is below the set point by apredetermined amount and for energizing the cooling means whenever thisaverage is above the set point by a predetermined amount.
 3. Apparatusaccording to claim 2 wherein there is a well in the radial periphery ofthe barrel and the first temperature sensing device comprises athermocouple mounted in the well, the heat exchange device is mounted onthe barrel, there is a well in the periphery of the heat exchange deviceand the second temperature sensing device comprises a thermocouplemounted therein and said control circuitry includes a null balancecircuit.
 4. Apparatus according to claim 3 wherein each of thethermocouples lie at different radial distances in a single planethrough and perpendicular to the barrel.
 5. Apparatus according to claim4 wherein the control circuitry is voltage regulating circuitry wherebythe amount of energy fed to the heating means increases as thedifference between the average indicated temperature and the set pointbecomes greater.
 6. Apparatus according to claim 1 wherein the heatexchange units include heating means and the means including controlcircuitry includes means for energizing the heating means whenever saidaverage is lower than a predetermined level.
 7. Apparatus according toclaim 1 wherein the heat exchange units include cooling means and themeans including control circuitry includes means for energizing thecooling means whenever said average is higher than a predeterminedlevel.